The well-known bow micrometer has a fixed anvil and a threaded spindle facing the anvil. The space between the face of the spindle and the face of the anvil is the measuring gap. The threaded spindle has 40 threads per inch so that each rotation represents 0.025 inch of measurement. The barrel around the spindle is calibrated to permit readout of measurements. Since many turns (40 turns) are required of the spindle to move it an inch, micrometers are provided in different measuring gap ranges. Furthermore, in order to reduce the number of micrometers and calipers needed for a full measurement range, they are provided with interchangeable anvils of different lengths.
Another problem arising from the use of interchangeable anvils is the fact that the measuring gap is always close to the spindle and to the micrometer bow. Sometimes the structure which is to be measured does not permit access in that way. For example, when a web is measured underneath a flange, it is desirable to be able to select the position of the measuring gap. Also, there are other configurations, such as pockets and steps, where it is desirable to check the thickness of all kinds of walls or the distance or depth of a variety of surfaces from the main surface. Thus, it is desirable to provide accessories which permit outside micrometers and calipers to have different sizes of selectable measuring gaps at a desired location between the anvil and spindle of the micrometer or measuring surfaces of the calipers.